In the past decades, the commercial application of cellular radio communications for cordless and mobile telephony has seen an enormous rise. Typically, these wide area cellular networks can be divided into two parts: a fixed part including an interconnected network of radio base stations, and a mobile or portable part including the mobile terminals that can access the network, for example, radio telephones. Each base station transmits control information on a control channel which can be used by the mobile terminals to access the network. Each radio base station in the network covers a restricted area, called the cell. The different base stations in the network are coordinated by way of the base station controllers (BSC). A frequency reuse pattern (fixed or adaptive) is applied to avoid interference in the transmissions from different base stations. Examples of these cellular systems include AMPS, D-AMPS, and GSM.
In private macrocell systems, the network part differs from the wide area cellular network equivalent. The private systems in general have to be much cheaper (since the system cost is shared among fewer users). In addition, the private systems typically cover an indoor environment which is less predictable than the outdoor environment (for example: walls, doors that open and close, corridors that act as wave guides). Therefore, in general, the radio base stations in indoor systems operate in a more autonomous way, determining themselves which channels to use for traffic and control (or beacon) information.
In business or office cordless phone systems like, for example, DECT, there can still be a certain degree of interaction between the base stations of a single indoor network. While the radio base stations in a business system like DECT are as autonomous as possible, they are loosely time synchronized through the network in order to allow for handovers from one base station to the other. Network functions are performed in a base station controller. For handover purposes, it is important that the beacons from different base stations arrive at the mobile terminal in a restricted time window to be scanned during idle frames of communications. In private residential systems such as, for example, a cordless phone, the radio base station of the cordless phone forms a single, private network which is only connected to the PSTN, and there are typically no communications or synchronization with other private, residential base stations (like those from the neighbors). In indoor radio systems, radio base stations themselves find the channels to operate on. These channels should preferably not interfere with other, nearby radio base stations. Therefore, a radio base station finds the channels with the lowest amount of interference (quietest channels) before it starts transmitting. Periodic measurements may be performed to assure that the base station remains on the least interfered channels.
Traditional analog radiotelephone systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as channels over which cellular radiotelephones (mobile terminals) communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 MHZ. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
The limitations on the number of available frequency bands presents several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequency bands available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
Another technique which may further increase channel capacity and spectral efficiency is time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Although communication on frequency bands typically occur on a common TDMA frame that includes a plurality of time slots, communications on each frequency band may occur according to a unique TDMA frame, with time slots unique to that band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots, and the European GSM standard, which divides each of its frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user's assigned time slots.
A channel in a TDMA system typically includes one or more time slots on one or more frequency bands. As discussed above, traffic channels are used to communicate voice, data or other information between users, for example, between a radiotelephone and a landline telephone. In this manner, each traffic channel forms one direction of the duplex communications link established by the system from one user to another. Traffic channels typically are dynamically assigned by the system when and where needed. In addition, systems such as the European GSM system, "frequency hop" traffic channels, i.e., randomly switch the frequency band on which a particular traffic channel is transmitted. Frequency hopping reduces the probability of interference events between channels, using interferer diversity and averaging to increase overall communications quality.
Included in the dedicated control channels transmitted in a cell are forward control channels which are used to broadcast control information in a cell of the radiotelephone system to radiotelephones which may seek to access the system. The control information broadcast on a forward control channel may include such things as the cell's identification, an associated network identification, system timing information and other information needed to access the radiotelephone system from a radiotelephone.
Forward control channels, such as the Broadcast Control Channel (BCCH) of the GSM standard, typically are transmitted on a dedicated frequency band in each cell. A radiotelephone seeking access to a system generally "listens" to a control channel in standby mode, and is unsynchronized to a base station or satellite until it captures a base station or satellite control channel. In order to prevent undue interference between control channels in neighboring cells, frequency reuse is conventionally employed, with different dedicated frequency bands being used for the control channel in neighboring cells, according to a frequency reuse pattern that guarantees a minimum separation between cochannel cells. Frequency hopping, which might allow denser reuse of control channel frequency bands, is typically not employed because an unsynchronized radiotelephone generally would have difficulty capturing a frequency-hopped control channel due to lack of a reference point for the frequency hopping sequence employed. Moreover, for private uncoordinated radio communications systems, a frequency reuse pattern cannot be used because each system operates independently of other potentially interfering systems.
In general, in radio communications control communications a downlink (from base to portable) for forward control channels and an uplink (from portable to base) are defined. A radio base station hears the portables' uplink information with its uplink receiver. In order to hear the downlink information sent by other base stations, the base station typically needs a downlink receiver as well. The uplink and the downlink can be distinguished by different frequencies, so-called Frequency Division Duplex (FDD), or by different time slots, so-called Time Division Duplex (TDD). Cellular systems typically use FDD as described above for downlink control channels. In order to measure other base stations, a downlink receiver would be built into the base station which adds costs. With the TDD scheme, the downlink may only be located at another time slot, so the downlink and uplink reception can be performed with the same receiver architecture. DECT, for example, uses the TDD scheme.
There are a number of reasons why, in certain applications, the usage of FDD is favorable above TDD. When the base stations are not time synchronized, a TDD scheme generally results in a mutual interference between uplink and downlink. In addition, because radio base stations are preferably placed at relatively high places in order to get line-of-sight to the portables, interference from base stations (to portables and other base stations) may be dominant. In FDD, uplink and downlink are completely separated in frequency and generally do not interfere with each other.
If in addition, private systems are considered that are based on cellular air-interface standards like GSM or D-AMPS, FDD may be applied for compatibility reasons. Therefore, in private radio communications systems applying FDD for distinguishing uplink and downlink, base stations typically determine which channel to operate on without knowledge of the transmissions from other, nearby radio base stations.
This problem relates particularly to the control or beacon channel of the base stations which transmits periodically in order to attach portables. For traffic channels, the system might be able to use the downlink receiver in the portable to derive knowledge about the interference situation locally. The downlink measurements made in the portable can then be transferred to the radio base station which can then select the optimal (duplex) traffic channel. For the beacon channel, this method generally is not applied, because the presence of a portable cannot be guaranteed when there is no traffic.
In uncoordinated private radio communication systems, mobile terminals and base stations may be unable to even establish communications access if radio beacon interference occurs. Such interference may occur between radio beacon transmissions of uncoordinated private radio communication systems which are located within an interference distance and transmit radio beacons in overlapping times and frequencies. In particular, since radio beacon transmissions are transmitted at fixed time intervals, they can mutually interfere for extended periods of time, effectively preventing mobile terminal access to the uncoordinated systems.